1. Field of the Invention
The present invention relates to a thin-film transistor provided with an oxide semiconductor thin film and a method of producing the thin-film transistor. The present invention further relates to devices, such as a display device, an imaging sensor and an X-ray digital imaging device, using the thin-film transistor.
2. Description of the Related Art
In recent years, thin-film transistors using an In—Ga—Zn—O (IGZO) oxide semiconductor thin film as a channel layer are actively developed. The IGZO oxide semiconductor thin film can be formed by a low temperature film formation process, exhibits higher mobility than amorphous silicon, and is transparent to visible light. Therefore, the IGZO oxide semiconductor thin film allows formation of a flexible transparent thin-film transistor on a flexible substrate, such as a plastic plate or a film.
Table 1 shows comparison of characteristics, such as mobility and process temperature, of various transistors.
TABLE 1LTPSLow-TemperatureOrganicOxidePoly Silicona-Si:HμC-Si:HTFTTFTMobility100<12-3<1-53-50Stability<11001-2301-2 ΔVTHUniformityAcceptableExcellentGoodAcceptableGoodFilm450300300RT-100RT-350FormationTemperature
Conventional poly silicon thin-film transistors can provide a mobility of about 100 cm2/Vs. However, the poly silicon thin-film transistors require a very high process temperature of 450° C. or more, and thus only can be formed on substrates with high heat resistance. Therefore, the poly silicon thin-film transistors are not suitable for providing large-area flexible thin-film transistors at low costs. Amorphous silicon thin-film transistors can be formed at a relatively low temperature around 300° C. and allow a wider selection of substrates than those usable with poly silicon. However, the amorphous silicon thin-film transistors can provide a mobility of around 1 cm2/Vs at best, and thus are not suitable for high-definition display applications. In view of low temperature film formation, organic thin-film transistors can be formed at a temperature of 100° C. or less, and are expected to be applied to flexible displays, etc., using a substrate with low heat resistance, such as a plastic film. However, the organic thin-film transistors have only achieved the same level of mobility as that of the amorphous silicon.
That is, a thin-film transistor that can be formed at a relatively low temperature around 300° C. or less and has a high mobility of around 100 cm2/Vs has not yet been accomplished.
As a method to improve the carrier mobility of a transistor, a HEMT (High Electron Mobility Transistor) structure has been proposed, in which different types of semiconductors having different electron affinities are joined and a quantum well is used as the channel of the transistor. With respect to the oxide semiconductor thin-film transistors, it has been reported that a device having the HEMT structure formed by sandwiching ZnO with ZnMgO provided a high mobility of 140 cm2/Vs (K. Koike et al., “Characteristics of a Zn0.7Mg0.3O/ZnO heterostructure field-effect transistor grown on sapphire substrate by molecular-beam epitaxy”, Applied Physics Letters, Vol. 87, 112106, pp. 112106-1-112106-3, 2005 which is hereinafter referred to as Non-Patent Document 1).
Further, as a thin-film transistor using the IGZO oxide semiconductor thin film, a thin-film transistor with an active layer which has a multilayer structure formed by IGZO films having different physical quantities has been proposed. Japanese Unexamined Patent Publication No. 2006-165529 (which is hereinafter referred to as Patent Document 1) discloses a field effect transistor with an active layer containing an amorphous oxide, where the active layer has a double-layer structure including a first area and a second area nearer to a gate insulating film than the first area, and an oxygen concentration at the second area is higher than an oxygen concentration at the first area. Patent Document 1 teaches that, with this structure, a higher electric resistance is provided at the active layer near the gate insulating film, and the channel is formed inside the amorphous oxide, thereby allowing reduction of leakage current.
However, Non-Patent Document 1 does not consider controlling the electron affinity by controlling the level of the oxygen concentration, as disclosed in the present invention, and all the HEMT structures disclosed in Non-Patent Document 1 are formed through single-crystal thin film epitaxial growth, such as MBE (Molecular Beam Epitaxy). In order to employ this type of growth method, it is necessary to minimize the lattice mismatch between the film and the substrate to a very low level and keep the substrate temperature during the film formation to a high temperature over 700° C. Therefore, this type of growth method allows very limited selection of substrate materials.
Further, although it is stated in Patent Document 1 that the invention of Patent Document 1 allows reduction of leakage current, it fails to provide a sufficient carrier density, resulting in insufficient mobility.